Add the vinegar, water and sugar to a pan and heat over a medium heat, stirring to dissolve the sugar.
Add the jalapenos and green chili (if using) to a blender or food processor and pulse to break down. Don't over mix, just pulse a few times, you don't want to make a paste.
Add the processed jalapeno to the pot and bring to the boil.
Simmer for 5 minutes and then take off the heat.
Mix together the cornflour and 6 tablespoons of water and pour into the pot. Stir for a minute or so to help thicken the sauce.
Notes
Pulse don't blend the jalapeños. If you over blend them, they will be too liquidy.
This sauce will thicken as it cools. Once cooled, if you prefer a thicker sauce, simply reheat it and stir in more cornflour and water.
Let the sauce cool completely before storing.
Nutritional values are based on one of 18 servings.